The invention relates to drawn parts made of steel plate, particularly for motor vehicle bodies, such as vehicle body outer skin parts and load-bearing structural parts, but also to comparable lightweight construction parts of different applications as well as to a corresponding manufacturing process.
Preferably high-quality drawn steel types are used nowadays in vehicle body construction for the exterior covering parts. Since, because of the low yielding point of the material, these steel types can be deformed very easily, minor damages during the manufacturing, but particularly during the use of the finished vehicle will necessarily lead to permanent deformations. Minor bumps, for example, when parking on parking lots, the impact of rocks, hail, or the like, result in dents which can be removed only at relatively high expenditures.
It is known (Krafthand Journal, Volume 4, Feb. 15, 1986, Pages 151 to 159) to use high-strength steel types in vehicle body construction for exterior covering parts of vehicle bodies which, although they require higher expenditures for repair work, are nevertheless less susceptible to damage. However, no detailed information is supplied there with respect to the recommended higher-strength steel types. Neither concrete materials, nor characteristic values, nor a pretreatment of the materials are mentioned. The different characteristic processing values to which reference is made there with respect to xe2x80x9cstandard sheet metalxe2x80x9d in comparison to xe2x80x9chigher-strength sheet metalxe2x80x9d indicate, because of the relative small difference, that the xe2x80x9chigher-strength sheet metalsxe2x80x9d are those which have an inherently higher material strength in the unhardened state.
German Patent Document DE 195 29 881 C1 suggests the deep drawing of vehicle body parts made of a hardenable spring steelxe2x80x94preferably in the hot statexe2x80x94and bringing the workpiece to spring steel hardness only in the finished drawn condition. This method of operation requires high-expenditure heatable deep-drawing tools and a quenching system integrated in the pressing facility, which is cumbersome and results in expectations of a high fault rate in the series operation with such a heterogeneous manufacturing using a hot deforming and a heat treatment.
In a contribution from the German journal Bleche Rohre Profile, 40 (1993), Page 906 and on, with the title xe2x80x9cNew Developments in the Manufacturing of Motor Vehicle Partsxe2x80x9d, the term xe2x80x9chigher-strength steel typesxe2x80x9d is also used for vehicle body steel plates, which, however, are steel materials which are not heat-treated and which, because of their alloy composition, inherently, that is, in the unhardened state, of a higher material strength than conventional sheet metal steels. In this context, the following steel types are mentioned: Age-hardening steel, Ti- and Nb-micro-alloyed steel as well as martensitic and bainitic polyphase steel. It is explicitly mentioned there that generally the deformability decreases as the strength rises. Problems in this respect are discussed there in detail.
It is an object of the invention to be able to manufacture in a simple manner drawn pieces of the initially mentioned type which have a high strength.
According to the invention, this object is achieved by means of the characterizing features of Claim 1 (drawn piece) and by those of Claim 6 (process). Accordingly, for drawn pieces, particularly for vehicle body parts, a spring steel is used in the heat-treated, specifically bainitized state, and this spring steel is deep-drawn or stretch-formed. This can be achieved surprisingly well despite the hardened metal plate by means of a corresponding tool design, press design and process forces. However, for this purpose, significantly higher process forces are required than previously. All process parameters required for the deformation have to be newly determined empirically, which can, however, easily be achieved within the scope of normal optimizing tests and as a function of the respective component.
Because of the fact that, during the deforming process, the spring steel can be stressed without failing by a significantly higher hold-down force and deforming forcexe2x80x94in comparison to conventional deep-drawn steel typesxe2x80x94, surprising large deforming degrees or deep drawing can be achieved. Because of the characteristic mechanical values of the hard spring steel plate, which are usually poor for a suitability for deep drawing, specifically a low uniform elongation and elongation at rupture, a low consolidation index, a low vertical anisotropy, a high deformability could definitely not be expected. Because of the nevertheless possible, high degrees of deformation, not only less deep-drawn outer skin pieces can be deep-drawn without any problems, but also load-bearing structural parts for which greater degrees of deformation are required. In the case of the latter, it is useful to superimpose a fine structuringxe2x80x94honeycomb or waffle structurexe2x80x94at least in slightly curved areas of the drawn piece, so that a high stiffness can be achieved here.
As the strength increases, that is, as the elastic limit rises, hardened metal plates generally lose deforming capacity. The measurable mechanical characteristic values, such as the uniform elongation or the elongation at rupture usually diminish as the strength of the plate bar material increases. This also applies to the bainitized spring steel recommended according to the invention. However, surprisingly, this material has unusually high characteristic deforming values xe2x80x9cvertical anisotropyxe2x80x9d and xe2x80x9cconsolidation indexxe2x80x9d, which are not customary for conventionally hardened spring steel. Because of the high characteristic deforming values of the bainitized spring steel recommended according to the invention, it is possible that very high process forces can be transmitted without cracks. The process forces occurring or to be used in this case may reach approximately ten to twenty times the process forces during the deep drawing of conventional deep-drawn steel. Because of these surprisingly high characteristic deforming values of the bainitized spring steel recommended according to the invention, the good deep-drawing results could be achieved in an interaction with the abnormal process forces.
In this context, it should be pointed out that the deep drawing or stretch forming of metal plate components makes significantly higher demands on the deformability of a material than other deforming processes, such as bending or punching. In the case of deformation-critical materials, the deep drawing or stretch forming is only slightly more difficult to control than the bending, because the drawing results in a surface enlargement with respect to the starting plate bar. Not every material which can easily be cold-bent is also easily suitable for use as a drawn material.
The heat treatment of the bainitizing of spring steel is known per se. In older German-language literature references, this type of heat treatment is also called isothermal hardening or austempering. It is also known that bainitized spring steel can be cold-deformed (compare, for example, a contribution from the technical journal Draht, 3 (1992), Pages 307 to 310). However, as far as the applicant knows, only bending or punching are known as deforming methods of such materials, but not deep drawing or stretch forming, for which a much higher degree of stretching and upsetting and mainly a multidimensional stretching and upsetting is demanded.
During the bainitizing, the steel has to be cooled from the austenization temperature and from the austenized condition very rapidlyxe2x80x94approximately within one secondxe2x80x94to a temperature which is clearly above the martensite starting temperature, thus, for example, to approximately 300 to 350xc2x0 C., for which salt baths or lead baths can be used. The steel must then be maintained at this temperature for a certain time which depends on the alloying of the steelxe2x80x94for approximately 2 to 10 minutesxe2x80x94until the austenite is transformed completely into a bainitic structure. Only then can the steel cool to room temperature, which can take place by allowing it to stand or by blower cooling.
The yield point of the hardened bainitized spring steel is approximately in the range of from 800 to 1,800 N/mm2, preferably 900 to 1,200 N/mm2; the tensile strength is by approximately 100 to 200 N/mm2 above it. Althoughxe2x80x94as mentioned abovexe2x80x94, the deforming capacity of a material decreases with its increasing yield point, of the bainitized spring steels from the PT group known to the applicant, the spring steel with the name PT120 with a tensile strength of 1,200 N/mm2 and a yield point of 1,000 N/mm2 is particularly recommended because, according to the applicant""s knowledge, it has a very good strength in this group combined with very good deforming characteristics. Spring steel may be used which is made of conventional carbon steelxe2x80x94, for example, of the DIN (standard) designations CK60 or C63xe2x80x94or of stainless alloyed spring steelxe2x80x94such as 67SiCr5xe2x80x94. The use of stainless steel plate is recommended in this case particularly because the expected durability of the less dent-susceptible motor vehicle body parts is higher than previously and because a rusting-through in the case of non-stainless steel types with a high durability and smaller wall thickness is possible earlier than previously.
The spring steel with the designation Ck60 (according to DIN) or 2 C 60 (according to Euronorm 83), which, for example, is recommended, has the following composition in weight percent:
Because of the high strength of the material, a metal plate which is clearly reduced in its thicknessxe2x80x94approximately 0.3 mm to 0.5 mmxe2x80x94in comparison to previously customary metal plate thicknessesxe2x80x940.8 mmxe2x80x94can be processed to form vehicle body parts. Simultaneously, the vehicle body parts according to the invention exhibit an improved denting behavior; that is, the tendency for a plastic deformation in the event of minor damage is clearly lower than previously. As a rule, minor indentations rebound elastically to their original shape. As the result of the smaller metal plate thickness, the vehicle body parts produced therefrom, while the denting behavior is improved and the overall strength is the same, can be designed to be approximately 20% to 60% lighter than previously.
Because of the high denting resistance of the drawn pieces according to the invention, these are of interest not only for outer skin parts. On the contrary, the present invention opens up new application possibilities under the aspect of a lightweight vehicle body construction. The previously thinnest vehicle body metal plates had a thickness of approximately 0.7 mm. As a result of the invention, metal plate thicknesses of approximately 0.3 mm definitely seem possible with respect to aspects of strength, particularly when a fine structuring is superimposed on less curved workpiece areas. Because of a metal plate thickness reduction by up to 0.5 mm or to approximately one third of the previous wall thickness which seems possible, considerable reductions of weight should be possible on the motor vehicle body structure which are at least equal to the use of light metals. If, in addition, a spring-hard stainless steel is used, the corrosion disadvantage of thin steel plates can also be compensated.